自有阶级以来，警察普遍存在于各个历史时期的各种类型的国家。
参考答案：对
Some college students are seen doing_____ work they can find to support themselves.
A．that
B．which
C．whatever
D．no matter what
作为X线照片影像的可供诊断的密度范围是（）
A.0.5～1.0
B.0.5～2.0
C.0.25～2.0
D.0.25～2.5
E.0.5～2.5
